Contrast control for television systems



Jul 22,1941. w. BU'NGER 2,250,293

CONTRAST CONTROL FOR TELEVISION SYSTEMS Filed July 23, 1938 gvf' T IO INVENTOR.

WA LTER BUNGE R A TTORNEYS.

Patented July 22, 1941 CONTRAST CONTROL FOR TELEVISION sYsrn s Application July 23, 1938, Serial No. 220,855 In Germany July 23, 1937 3 Claims. ((31. it's-v.2)

This invention relates to means for contrast control for television systems.

The maximum range of contrast obtainable in television images at present, when employing cathode-ray viewing tubes, is approximately 10 to 1, that is, ten different tone values canbe distinguished by the human eye. The quality of an image so obtained corresponds to that of a well-printed newspaper picture, while a photographic image shows 50 tones and a good projected image 100 tones. If objects are transmitted which have a greater range of contrast than 10 to 1, it is unavoidably necessary to adjust the range of contrast in such a manner that the reproduced image deviates as little as possible from the original image. As the picture contents will vary during transmission, it is necessary to take this into consideration. Previously the degree of amplification'of the modulating stage has been manually regulated in such a manner that the total range of contrast was kept. within the linear portion of the amplifier tube characteristics. If the picture contents change due, for instance, to the change in scene to be transmitted, the degree of amplification must be readjusted.

The conditions at the receiving tube are as follows. The available range of contrast lies between two limits. One limit is determined by the voltage applied to the control grid of the cathode ray viewing tube for reproduction of black. Absolute black, however, or total absence of light, will usually not be obtainable. Thebther limit is determined by that value of the cathode-ray beam intensity at which a spreading of the fluorescent spot occurs. Thus it may be seen that the range of contrast available at the viewing tube lies between the two above-mentioned limits.

Assuming an image is to be transmitted which possesses a range of contrast of only ten tones, the darkest of which may be represented by the black level of the carrier, the degree of amplification must be adjusted to a comparatively large value in order to reproduce this image faithfully. If the image to be transmitted is subjected to brighter illumination, the range of contrast will also change and may, for instance, increase to 100 tones. If the degree of amplie fication were not readjusted, the first ten tones, that is, the darkest portions of the image, would be faithfully reproduced, but all bright portions would overload the cathode-ray viewing tube and would be reproduced with uniform brilliancy, so that no detail would be visible in the bright portions of the image. Therefore, it is necessary to decrease the degree of amplification in such a case, so that the peak-to-peak value of the received picture signal lies within the linear portion of the characteristics of the cathode-ray viewing tube. In doing so, a range of ten tones at the transmitting end will be compressed into one tone at'the receiving end, in the case of an image to be transmitted having tones and an available range of contrast at the viewing tube of 10 to 1.

Whereas the adjustments of the degree of amplification have been operated manually in the past, it is the object of this invention to provide an automatic means for this purpose. Broadly considered, this invention provides for making use of a signal correspondingto the average brightness of the image, and provides for'utilizing this signal tovary the position of the operating point of a tube with exponential characteristics in a picture-signal amplifier, to the grid of which tube the picture signal is'applied. It is possible to use the signal which corresponds to the average brightness of the image,- or, as it is usually called, the direct current com-' ponent of the picture signal, for this purpose, because a corresponding change in average picture intensity occurs with each change in range of contrast. It is assumed hereby that it is desirable todevote more of theavailable contrast to the more extended portions of the image, while the, loss in contrast of reproduction of smaller portions of the image at the receiving end is allowable, without impairing the quality ofthe reproduced image. 3 While this invention is not limited to any particular type of image analyzing tube as long as the direct-current component is made available in somemanner, the invention will now bedescribed in further detail in connection with an analyzing tube of the type of the Farnsworth Image Dissector, as described in U. S. Patent 1,773,980, issued August 26, 1930, U. S. Patent.

1,970,036, issued August 14, 1934, and U. S. Patent 1,986,330, issued January 1, 1935. These tubes inherently produce a signal corresponding to the average intensity of the image. This signal is used to vary the bias applied to one or more tubes possessing exponential characteristics and which are incorporated in a picturesignal amplifier.

Referring to the drawing, Fig. 1 shows an embodiment of this invention, reduced to the simplest term-s. Fig. 2 shows another embodiment of this invention, also reduced to the simplest terms.

In Fig. 1, tube I is a Farnsworth image dissector wherein focusing and deflecting means are not shown. Magnetic focusing can be obtained by surrounding tube I with a coaxial coil through which a direct current is caused to flow. Deflection is produced by a coil system surrounding the tube through which deflecting currents are applied, according to the teachings of the abovementioned Farnsworth Patent 1,986,330. The optical image is focused upon a photo-electric cathode 2. The tube I also contains a cylindrical accelerating electrode 3 and a tubular anode 4 possessing an aperture 5. An electron multiplier system 6a is disposed inside the tubular anode 4. The picture signal may be taken from the electron collector 6 of the multiplier and is applied to the grid of an amplifier tube 8 through a condenser 1. Tube 8 has exponential characteristics. Resistor 9 is disposed in the electrical path between cathode 2, voltage source I6, and tubular anode 4. Resistor 9 is bypassed by condenser I0, and is grounded at one side at II. The entire photocurrent produced at the photocathode 2, less the minute portion which enters through aperture into tubular anode 4, will then flow through resistor 9 and thereby cause a voltage drop corresponding to the average brightness of the image. The voltage developed across resistor 9 is applied to the cathode of tube 8 in such a manner that a decrease in average brightness of the image will cause a decrease in negative grid bias at tube 8, thus increasing the degree of its amplification.

A further embodiment of the invention is shown in Fig. 2. Tube I is a Farnsworth image dissector, wherein focusing nd deflecting means are likewise omitted for the sake of simplicity. Picture signal collector 6 of the electron multiplier system 6a is directly coupled to the grid of tube I2, which has exponential characteristics. Resistor I3 is disposed in the electrical path between photocathode 2, voltage supply source I6 and tubular anode 4. Resistor I3 is bypassed by condenser I4 and connected to ground at I5. Again the entire photocurrent produced at photocathode 2, minus the minute portion entering at aperture 5 in tubular anode 4, flows through resistor I3, thus producing a voltage drop proportional to the average brightness of the image. Resistor I3 is connected to the grid of tube I2 through resistor I6, thus applying a bias to this grid equal to the voltage drop across resistor I3. A decrease in average brightness of the image again causes a decrease in negative bias on the grid of tube I2 and thereby an increase in its amplification.

While I have described my invention in connection with the Farnsworth image dissector, as

described in the above-named U. S. patents, I do not wish to be limited to the use of such. It may be readily seen that my invention is adaptable for use with any other pickup tube, provided that a signal proportional to the average brightness of the image, commonly referred to as the direct-current component of the picture signal, is made available in some manner.

What I claim is:

1. A television system comprising an image dissector tube possessing a photocathode of picture area adapted to be illuminated by an optical image, an anode adapted to receive electrons from substantially the entire illuminated area of said photocathode, a picture-signal collector adapted to receive electrons from successive elemental areas of said photocathode only, and electron-controlling means; a resistor shunted by a capacitor connected in series with a source of high voltage between said photocathode and said anode; a picture-signal amplifying tube possessing exponential characteristics and having a grid and a cathode; means for applying the direct-current voltage developed across said resistor between said cathode and said grid; means for utilizing the electrons received by said picturesignal collector to develop a picture signal, and means for applying said picture signal to said grid.

2. In a television system, comprising an image dissector of the type described possessing a photocathode of picture area, an apertured tubular anode, a picture-signal collector within said anode and electron control means, a resistor shunted by a capacitor conductively connected between said photocathode and ground, a source of high voltage connected between said anode and ground, a resistance-capacitance coupling between said picture-signal collector and the grid of a picture-signal amplifying tube possessing exponential characteristics, said grid being grounded, and a conductive connection between the cathode of said amplifier tube and said photocathode.

3. In a television system, comprising an image dissector of the type described possessing a photocathode of picture area, an apertured tubular anode, a picture-signal collector within said anode and electron control means, a source of high voltage connected between said photocathode and ground, a first resistor conductively connected between said anode and ground, a conductive connection between said picture signal collector and the grid of a picture-signal amplifier tube possessing exponential characteristics, the cathode of said tube being grounded, and a second resistor conductively connected between said picture-signal collector and said anode.

WALTER 'BiiNGER. 

